1. Field of the Invention
The present invention relates to the field of digital demodulators and to at least amplitude demodulators.
The present invention applies, for example, to electromagnetic transponder systems, be it on the terminal or on the transponder side. The present invention more specifically applies to transponders having no autonomous power supply (for example, contactless smart cards) and to autonomous terminals (battery-supplied), for example hand held terminals.
2. Discussion of the Relate Art
FIG. 1 very schematically shows in the form of blocks a conventional example of the architecture of a contactless smart card to which the present invention applies.
Such a smart card or transponder is intended to communicate with a read/write terminal (not shown) and draws the power necessary to the supply of the circuits that it comprises from a high-frequency field radiated by the terminal.
A transponder essentially comprises an oscillating circuit formed of an inductance L in parallel with a capacitor C in charge of sensing a high-frequency field (for example, at 13.56 MHz). A rectifying component 2, for example a fullwave rectifier, is connected to terminals 3 and 4 of the oscillating circuit and provides a D.C. voltage, filtered by a supply capacitor Cf, to application circuits 5 (IC) of the transponder. A linear regulator 6 may be interposed between the rectified output of bridge 2 and the power input terminal of circuits 5.
The transmission of information from a read/write terminal (not shown) to a transponder is performed by modulating, at the rate of a sub-carrier (for example, at 53 kHz) the remote-supply carrier (for example, at 13.56 MHz) according to the code of the message to be transmitted. This modulation is an amplitude modulation with a non-zero modulation index. On the transponder side, an amplitude demodulator 10 (AM DEMOD) is connected on one 3 of the terminals of the oscillating circuit upstream of the rectifying component and provides circuit 5 with the demodulated information.
The transmission of information from the transponder to the terminal is performed by means of a reverse modulation at the rate of a sub-carrier (847.5 kHz). This reverse modulation consists of modifying the transponder load on the electromagnetic field radiated by the terminal. For example, a series association of a resistor R and of a switch K controlled by circuit 5 is connected in parallel with oscillating circuit 1. Capacitive reverse modulation systems may also be found. On the terminal side, an amplitude and phase demodulator is provided to exploit the data transmitted by the transponder.
The present invention more specifically relates to the forming of the amplitude and possibly phase demodulator of an electromagnetic transponder or of a read/write terminal.
The analog-to-digital conversion of the transmitted information is performed either downstream of the demodulation which is then analog, or downstream of a digital decoding. The present invention more specifically relates to the case of a demodulation which requires a sampling of the signal extracted from the oscillating circuit before driving the demodulator. Generally, and for simplification, the sampling is then considered as belonging to the demodulator. By convention, the demodulator portion located downstream of the sampler, which provides the demodulated information, that is, the baseband (carrierless) digital data will be called the “digital demodulator”.
FIG. 2 shows a conventional example of a demodulator providing, from an amplitude-modulated signal, two data I and Q enabling determining the amplitude and the phase of the sub-carrier signal, for example, in a QAM demodulation.
The received signal (for example, sampled from terminal 3 of the transponder) is submitted to an analog-to-digital conversion or sampling (block 11, CAN) with a sampling frequency fs before driving a digital demodulator 12 (DAM DEMOD). The samples provided at the output of demodulator 12 are on the one hand submitted to a multiplication by cos(ωt), where ω represents the pulse of the sub-carrier and t represents time, before crossing a low-pass filter 14 (LPF) to provide information I, and on the other hand multiplied (multiplier 15) by sin(ωt) before crossing a second low-pass filter 16 (LPF) providing information Q.
FIG. 3 illustrates what information I and Q amount to on a trigonometric circle of radius A, where A designates the amplitude of the sub-carrier signal. Knowing I and Q, the values of amplitude A and of phase φ are determined by applying the following relations:A=√{square root over (I2+Q2)}; andφ=Arctan(Q/I).
For the demodulator of FIG. 2 to be operative, sampling frequency fs must in practice be four times greater than the frequency of the carrier conveying the data. Theoretically (Shannon theorem), the sampling frequency could be equal to four times the carrier frequency, provided that the signal to be demodulated is in phase with the carrier, which is in practice very seldom the case. In particular, for electromagnetic transponders, a phase shift necessarily appears due to the distance variation between the transponder and the terminal so that this condition cannot be fulfilled. Conventionally, a sampling frequency greater than four times the carrier frequency is used to be sure to obtain information I and Q in reliable fashion for any sample.
In the example of electromagnetic transponders where the carrier frequency is 13.56 MHz, this requires a sampling frequency greater than 54.24 MHz. Now, the higher the sampling frequency, the greater the power consumption of the circuit implementing the conversion. Further, since the power consumption is proportional to the square of the supply voltage, this requires using high quality technology to avoid worsening power consumption performance, which results in an expensive technology.
The same problem occurs in a read and/or write terminal, the power consumption of which is desired to be minimized, especially if it is autonomous.
Currently, one solution to decrease the sampling frequency consists of performing an analog demodulation, then converting the demodulated signals to digital. This however poses many problems, especially for the impedance matching and the balancing of paths I and Q. It is thus generally preferred to digitize, as soon as possible in the receive chain, the signals to be exploited.